The present invention relates to a pressure measurement system and, in particular, to a pressure measurement system which incorporates sensing and reference capacitor networks energized by an external source and a feedback loop to generate oppositely-phased signals which are summed to produce an error signal representative of the difference therebetween. The error signal is demodulated and digitally processed through the feedback loop to null the error signal and generate a digital signal representative of pressure.
Pressure measuring systems incorporating this basic configuration are disclosed in U.S. Pat. No. 4,322,977 entitled, "Pressure Measuring System" to Robert C. Sell, John R. Sheler and John M. Juhasz, and U.S. Pat. No. 4,449,409, to Frank J. Antonazzi, and other patents referenced therein. These patents are assigned to the assignee of this invention. Characteristically, this general type of pressure measuring system is rugged and provides for pressure measurement to a high degree of accuracy. Such systems find broad application in such rigorous environments as fuel control systems for gas turbine engines. In such applications, it is common for the pressure sensors, a central processing unit, power supplies and the like to be located in various parts of an aircraft. From the above-referenced United States patents it will further be seen that these systems incorporate a phase sensitive demodulator to receive, demodulate and amplify the error signal subsequently used to generate a nulling signal and provide a digital indication of pressure. Because the demodulator is phase sensitive, any phase shift between the error signal and the excitation source utilized for the phase sensitive demodulator resulting from variations in wiring from system to system can cause variation in the pressure measurement and careful matching of units becomes necessary when units are interchanged. Accordingly, it is desirable to provide a pressure measuring system in which this problem is eliminated.
Broadly, the present invention is a pressure measuring system which includes at least one pressure sensitive capacitor and a reference capacitor. The pressure sensitive capacitor is connected to a source of excitation voltage and the reference capacitor is excited by an oppositely phased signal from a feedback loop. The output signals from the two capacitors are summed to produce an error signal which is demodulated by a phase sensitive demodulator to generate a signal which is applied to a digital computing circuit to generate the feedback signal applied to the reference capacitor. Simultaneously, the digital circuit generates a digital number or value corresponding to the feedback signal required to null the capacitance circuit and, correspondingly, a signal which corresponds to pressure. The circuit is characterized in that it includes a phase sensitive demodulator reference voltage source which generates a source signal for the phase sensitive demodulator precisely in phase with the capacitor excitation signal. The phase sensitive demodulator excitation source is located adjacent to the phase sensitive demodulator. This reduces wire requirements between the central processor and transducer module. This in turn assures phase coherence and interchangeability of the pressure transducer modules without special matching.
It is therefore an object of the invention to provide an improved pressure measurement system.
It is another object of the invention to provide such a system in which the effect of a phaser difference in the excitation signals for the pressure sensing capacitor(s) and phase sensitive demodulator is eliminated.
Still another object of the invention is to provide a phase shift demodulator circuit for use in a pressure measurement system wherein the excitation signal for the phase sensitive demodulator is derived directly from the input to the pressure sensing capacitor.